The present invention relates to a magnetic disc substrate which uses a thermoplastic norbornene resin. More particularly, the present invention relates to a magnetic disc which can conduct the position control of the magnetic head with high accuracy.
In a magnetic recording medium for recording and reproducing digital information by a magnetic head, for example, a magnetic disc, a servo mark is written to make the magnetic head follow the recording track and to give an address information. In order to carry out a tracking of the magnetic head with high accuracy (hereinafter to control the position so as to follow the record track precisely in a predetermined address is referred to as "tracking"), the resonance frequency of the magnetic disc must be elevated to higher than the servo band. Therefore, materials having high modulus of elasticity, such as alin, glass or ceramics, have been used as a magnetic disc substrate. At the time of recording the data, it has also been carried out to write a position signal to the magnetic disc by writing a servo mark simultaneously by a servo writer.
However, as the width of the recording track is becoming narrower as the density of records becomes high, high positional precision of said position signals becomes necessary. And in the conventional writing method of the position signal, it becomes necessary to maintain high positional precision between the servo writer and the mechanism portion of the magnetic recording device. And there is a problem that since it requires extremely difficult technology, the device becomes expensive.
Therefore, a method has been proposed wherein the positional precision of the position signal is increased by preforming a servo mark in the magnetic disc. For example, there can be mentioned a method in which a servo mark is formed by etching or by non-magnetizing the magnetic layer, or a method in which a magnetic disc substrate is molded by a molding technique and the like, and at the time of molding, an uneven servo mark is formed.
However, in said method to form a servo mark by etching or non-magnetizing a magnetic layer, there is a problem in that the production process of the magnetic disc becomes complicated thereby making the industrial production difficult.
On the other hand, a method to form an uneven servo mark by molding a magnetic disc substrate by a molding technique is excellent productivity-wise and industrially advantageous. However, if the plastic used as a substrate is a polycarbonate, a polymethylmethacrylate the like used for the optical disc substrate, the deformation due to moisture absorption is too big, and it is not suitable. Furthermore, a polymethylpentene, a polystyrene or the like having a low rate of moisture absorption is not preferable because the deformation of the substrate with the crystallization proceeding after the formation is too big since it is a crystallized plastic, or the heat resistance is not sufficient.
Furthermore, since the plastic has a low modulus of elasticity, the resonance frequency of the magnetic disc becomes low, thereby causing a problem in that the resonance frequency larger than the servo band cannot be obtained. Thus, any proper materials suitable as a magnetic disc substrate have not been found in the conventional plastics.